I coach Walter who is blind and Caroline who had severe head trauma and coma and has very limited motor response/reflex, etc.
I treat them as I would an Oly candidate, indeed until Walt injured himself, he was headed to Paralympic camp and more to their ages and gender than to
their adaptive challenges.
This forum has a lot of posts on my work with both of them.
off the top of my head, I think your rower will be more effective rowing starboard, and shouldn't have too much trouble feathering outside hand.
A lot of ppl are feathering outside hand already, we had discussion about that here. but he'll have more leverage on the handle with the outside arm.
I think it would be a fun challenge to try to engineer some sort of prosthetic
that would allow him to apply pressure with his inside shoulder during the drive before the outside arm breaks. Something flexible in a way that will
resist stiffly on the early drive but then flex easily so as not to resist
the pull with outside hand. No idea how to engineer that, but I think it would make him more effective in the shell. Your rower would have some ideas, I bet, now that he knows a little about rowing.